CRUCIBLE COATING FOR PREPARATION OF U AND P ALLOYS CONTAINING Zr OR Hf

ABSTRACT

A method of limiting adverse oxygen contamination in the preparation of uranium and plutonium alloys containing zirconium or hafnium, in which the metals are melted and alloyed in a graphite crucible coated with niobium carbide or yttrium oxide.

I United States Patent 1151 3,660,075

Harbur et a1. [45] M 2, 1972 [5 CRUCIBLE COATING FOR 1 References Cited PREPARATION OF U AND P ALLOYS UNITED STATES PATENTS CONTAINING ZR OR HF 2,548,897 4/1951 Kroll ..75/10 R [72] lnventors: Delbert R. Harbur; John W. Anderson, 2,778,730 1/ 1957 Spedding... ..75/63 both of Los Alamos; Richard K. Money, 2,822,260 2/1958 Feder ..75/63 Espanola, all of N, Mex. 3,019,102 l/l962 Saarivirta... ..75/135 3,109,730 11/1963 Zegler.... ..75/63 ASSlgl'lCCI The UIIIICd States Of America as p l 0 R by Ammic 3.443.912 5/1969 Silver ..23/346 Energy Commlssm" 3,017,286 1/1962 Kane ..1 17/119 22 pn Oct 1 19 9 3,138,453 l/l964 Foster ..75/84 1 PP 867,096 Primary ExaminerWinst0n A. Douglas Assistant ExaminerPeter D. Rosenberg 52 us. 01 ..75/65, 75/10, 75 34,

l17/5.1 [51] Int. Cl ..C22b 7/00, C22d 7/06, C22b 7/00 [57] ABSTRACT [58] Field of Search ..75/65, 57, 10, 84, 93-96, A met o o l t ng a rse o ygen contamination in the 75 35; 143 2 147 130 95 1 19; 23 344; preparation of uranium and plutonium alloys containing zir- 117 5 conium or hafnium, in which the metals are melted and a1- loyed in a graphite crucible coated with niobium carbide or yttrium oxide.

1 Claims, No Drawings CRUCIBLE COATING FOR PREPARATION OF U AND P ALLOYS CONTAINING ZR OR HF The invention described herein was made in the course of, or under, a contract with the US. Atomic Energy Commis- 81011.

This invention relates to a method of limiting oxygen contamination in alloys prepared by induction melting, and more particularly to the limiting of such contamination during the preparation of U-Pu-Zr and Pu-Hf alloys.

A great many alloys containing fissile metals have been considered as possible fuels for nuclear reactors. Alloys of U-Pu- Zr and Pu-Hf have been found to exhibit considerable potential in this respect. The advantages of preparing fuel alloys such as these by induction melting and alloying in a bottom pour crucible followed by direct casting are well known. However, a problem that has existed heretofore has been the high affinity shown by certain of the melting materials (e.g., zirconium and hafnium) for oxygen. The problem has been particularly severe at the temperatures (1,400 C. and higher) required, for example, to prepare a U-Pu-Zr alloy containing more than weight percent of zirconium. Oxygen contamination on the order of 1,000-1 ,500 ppm during the preparation of fuel alloys has not been uncommon. Unfortunately, oxygen in even much smaller concentrations frequently adversely affects the physical properties of the fuel alloys.

Heretofore, oxygen contamination of uranium and/or plutonium alloys which could be prepared below 1,400" C has been kept at acceptable levels through the use of graphite, alumina, magnesia, zirconia, or thoria crucibles. For certain alloys special refractory coatings such as magnesium zirconate, thoria, and beryllia were required on crucibles and molds. However, as alloying temperatures of 1,400 C and higher came to be required, it became apparent that crucibles made of or protected by these materials were inadequate to protect against unacceptable levels of oxygen contamination.

It is therefore an object of this invention to provide a method of limiting oxygen contamination during the preparation of fuel alloys by induction melting and alloying.

It is a further object of this invention to provide a method of limiting oxygen contamination in the preparation of uranium and/or plutonium alloys containing zirconium or hafnium.

It has now been found that oxygen contamination can be effectively prevented if the fuel alloys are prepared in graphite crucibles coated with a thin layer of niobium carbide. A layer only a few mills thick will suffice. The niobium carbide may be deposited on the graphite, for example, by a vapor phase process using niobium pentachloride at elevated temperatures.

The use of niobium carbide to protect graphite against structural attack is known in the art. US. Pat. No. 3,017,286 discloses that a niobium carbide coating is useful for preventing corrosion or erosion of graphite structures exposed to gases at high pressures, velocities, and temperatures. It is also suggested in that patent that various other graphite articles as, for example, crucibles, may be coated with niobium carbide to improve chemical resistance or the like. There is nothing in that disclosure, however, that would indicate that a NbC coating on a graphite crucible is in fact effective to prevent oxygen contamination in fuel alloys prepared at temperatures of l,400 C or higher, especially if the alloy contains a component, i.e., zirconium or hafnium, with a high afi'mity for oxen. The present inventors have now discovered that a niobium carbide coating on graphite crucibles is virtually inert to molten U-Pu-Zr alloys. Homogeneous U-Pu-6Zr to U-l5Pul5Zr alloys (where values for Pu and Zr are in weight percent) can be prepared by inductively heating the feed metals in vacuo to 150 to 200 C above the melting point of the alloy,

that is, to l,400-l ,500 C, in a NbC-coated crucible equipped with a NbC-coated graphite stopper rod. The molten metals are mechanically stirred with a NbC-coated graphite stirrer for 30 minutes and the alloy is then bottom cast into an apprgplriate mold, e.g., an ambient temperature aluminum mold. e content of a U-Pu-Zr alloy thus produced is between 40 and 120 ppm, which is the nominal range present in the feed materials. For example, in a U-l5Pu-l2Zr alloy the oxygen contents of uranium, plutonium, and zirconium used as feed materials were respectively 50, 20, and 170 ppm. The oxygen content at the top of a 0.75-inch diameter and 14-inch long billet of the alloy was found to be 110 ppm, while at the bottom it was ppm.

A typical Pu-Hf alloy, i.e., Pu-2Hf, prepared by induction melting in a NbC-coated graphite crucible showed an oxygen content of 60 ppm.

Alternatively it is possible to prepare U-Pu-Zr alloy by first arc melting a U-Zr alloy and then induction melting the U-Zr binary alloy with high purity plutonium, utilizing a Y,0,- coated graphite crucible, stirrer, and stopper. In a U-lSPu- 12Zr alloy prepared in this fashion, the oxygen contents of the U-Zr binary and the plutonium used as feed material were respectively and 20 ppm. The oxygen content of a 0.75- inch diameter by l4-inch long billet of the alloy was 270 ppm at the top and 220 ppm at the bottom.

The Y O coating may be applied to a graphite crucible by spraying colloidal Y O in a K SiO solution onto the desired surface using an inert-gas spray gun. The surface of the part to be coated is heated to to 200 C on a hot plate, and thin coats (two to 3 mills thick) are then sprayed on. Any loose particles of Y O can be rubbed off the surface using steel wool.

Once applied, the Y O coating has excellent thermal shock resistance and good abrasion properties, and is un-wetted by the molten U-Pu-Zr alloys. This lack of wetting by the melt is a distinct advantage over the NbC coating in that a Y O coating can thus be more readily reused. However, there is a pickup of about 100 ppm of silicon and oxygen from the Y O coating by the melt, whereas there is no detectable pickup of these adverse impurities when the NbC coating is used on graphite crucibles in the alloying process.

Heretofore, attempts to coat graphite crucibles with Y O have been unsuccessful due to a lack of adhesion between the Y O and the graphite. As a consequence, alloys prepared in Y O -coated crucibles were readily contaminated with Y O However, in the present embodiment the K SiO serves as an effective binder, holding the 0 to the graphite.

It should be understood that the utility of the invention herein disclosed is not restricted only to limitation of oxygen contamination in fuel alloys prepared by induction melting. Contamination by crucible materials themselves is strongly inhibited. Thus, the niobium and carbon concentrations in four different U-Pu-Zr castings prepared in NbC-coated crucibles were about 20 and 200 ppm respectively. Since the carbon content of this alloy significantly affects its machinability, the ability to keep carbon contamination to a minimum by use of this coating represents a manifest improvement in the induction melting method of preparing alloys of this type. If a Y O coating is used instead, there will be little or no carbon contamination; however, as noted, there will be a pickup of about 100 ppm of silicon and oxygen.

What we claim is:

1. A method of preventing adverse oxygen contamination in the preparation of fuel alloys selected from the class consisting of U-Pu-Zr and Pu-Hf which comprises inductively heating the feed metals to 150 C to 200 C above the melting point of the alloy in vacuo in a graphite crucible coated with a thin layer of yttrium oxide. 

